1. Field of the Invention
The invention relates to a method for reading a bar code extending into the depth of a substrate by means of optical interference, the bar code being represented by an area with marks in the substrate which is partly transparent to electromagnetic radiation.
2. Description of the Prior Art
A standard, usually printed bar code comprising a plurality of parallel lines or bars for machine readable article identification is generally known. A deep or depth bar code is understood to be one in which the bars are not arranged on or parallel to the marked substrate surface but are instead perpendicular to the surface in different depth planes of the substrate material.
For an exemplified explanation a partial volume of the substrate with a characteristic width D (e.g. edge length, diameter) and height H (perpendicular to the substrate surface) is considered as the marking area having the deep bar code. The area can be subdivided into N layers of identical thickness dH in each case, so that N.dH=H. Each individual layer may be marked or unmarked and the mark at least consists of a change in the optical characteristics compared with the unmarked substrate influencing the backscattering or reflection of light in at least one, preferably non-ionizing spectral range. On determining the positions of the marked layers with a resolution better than the layer thickness dH, this gives a bar code of length N bits.
It is advantageous when using deep bar codes that it is possible to accommodate a larger number of deep bar codes on substrates having a limited surface, because an individual bar code only requires a cross-sectional surface of approximately D2<1000 μm2. This e.g. makes it possible to record additional sample or specimen information or a detailed analytical history on the sample or specimen substrate. If a plurality of deep bar codes are preferably positioned in juxtaposed manner in such a way that linear movements of the reader or substrate permit the sequential reading of the bar code, it is possible to store in simple manner and with limited space a large data quantity (1000 deep bar codes per square millimetre). The individual reading processes remain independent of one another. In particular, the deep bar codes can be recorded at different times and with different writing or recording devices.
Another advantage of deep bar codes is the increased security against manipulation. With the naked eye a deep bar code appears in the visible range as a neutral “fuzzy spot”. Unlike a conventional bar code, without a reader it cannot be detected and translated into a number string. Thus, at least suitable devices are needed for reading and writing in order to be able to carry out specific manipulations, which would often fail as a result of the cost involved.
Numerous different methods can be used for writing or recording such deep bar codes. It is e.g. possible to produce labels from a plurality of superimposed, glued film layers with a differing transparency or refractive index, the sequence of the transparency change containing the bar code. In the case of semiconductors or other crystal substrates the bar code could be preset during the epitactic growth by a controlled modification of the material provided or the growth conditions.
A particularly interesting possibility for producing complete falsification-proof (because irreproducible) deep bar codes consists of introducing scattered particles into a hardening, optically transparent matrix, e.g. cast resin. The once fixed, precisely measured distribution of the scattered particles in the marking area represents a unique code, which cannot be precisely copied by similar procedures. Possible uses of such unique “number plates” are in the noninterchangeable marking of equipment, whose movement and use scope is subject to strict controls, such as military vehicles and weapon systems.
So-called internal engraving with laser light is suitable for glass substrates. Using brief laser light flashes, precisely located volumes are damaged in a selectable depth beneath the glass surface in such a way that in said volumes the substrate significantly loses transparency. It is already possible to engrave symbols visible with the eye in the glass without the glass surface suffering. Uses of this technology are the marking and archiving of biological and medical specimen carriers, e.g. for high throughput screening (HTS), which must not be contaminated by deposits on an engraved surface.
Independently of the details of the recording process, the problem arises of rapidly reading out with limited apparatus costs a deep bar code recorded in a marking area on a substrate, and this also constitutes the problem of the present invention.